During nanofabrication processes, hydrofluoric acid (HF) is used to et in water)

Question

During nanofabrication processes, hydrofluoric acid (HF) is used to et in water) etches silicon dioxide from wafers. However, concentrated HF (typically 49 wt % dioxide too quickly. Therefore, a buffered oxide etch solution is co controllable et solution (49 wt % HF in water). At 25°C, Ka of HF is 6.6 × 10-1 ne NH4F and 1.15g/mL for HF. HF in water) etches silicon mmonly used for more ching. It comprises a 6 L NH4F solution (4@wt % NH4F in water) and I LH . The density is 1.11 g/mL for Calculate the pH of this buffered solution at 25°c

Explanation / Answer

Mass of NH4F in the solution = Density*Volume = 1.11*6000 = 6660 g

Moles of NH4F in the buffer = Mass/MW = 6660/37 = 180

Mass of HF in the solution = Density*Volume = 1.15*1000 = 1150 g

Moles of HF in the buffer = Mass/MW = 1150/20 = 57.5

For HF, pKa = 4-log(6.6) = 3.18

Using Henderson Hasselbach equation:

pH = pKa + log(moles of salt/moles of acid)

Putting values:

pH = 3.18 + log(180/57.5) = 3.675

Hope this helps !

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